In devices accessible to a wired or wireless network, a sleep state is provided for reducing the period of an active state of each of the devices to considerably decrease power consumption.
Representative examples of network-accessible devices, which provide the sleep state, include cellular phones.
Since cellular phones receive an incoming call signal in a non-operating state, the cellular phones periodically check whether the incoming call signal arrives. Also, as a cellular phone moves, a camping cell is updated, and a position of the cellular phone is registered in a network, whereby it is unable to maintain the sleep state continuously. In addition, since an interrupt (for example, a key sense interrupt) that occurs when a cellular phone transmits an originating call signal is required to be processed within time delay that does not cause inconvenience to a user, it is unable to maintain the sleep state continuously.
Therefore, much development has been made for technology that maintains the sleep state of a cellular phone for as long as possible to reduce power consumption, and thus increases a call waiting time of the cellular phone.
Recently, due to the explosive increase in wireless traffic, various access methods are being provided for offloading of limited traffic in cellular mobile communication, and Wi-Fi is attracting much attention as a representative method of the various access methods.
In a wireless LAN, a wireless local area network (LAN) access point (AP) is installed at a wired network termination, and by providing communication between the wireless LAN AP and a wireless LAN device, traffic that uses cellular mobile communication is effectively distributed to a wired network. However, the wireless LAN still adds to the traffic load of a wired network.
To solve such a problem, application of direct communication technology that provides communication in a limited space without using the wired network and a wireless network is increasingly expanded. Examples of the direct communication technology include Bluetooth technology, which is performed with low power at a very close distance, and Wi-Fi P2P technology that uses wireless LAN technology and provides direct communication.
Hereinafter, requirements that are provided for the sleep state in a wireless network in consideration of a characteristic of the direct communication will be described with the Wi-Fi P2P technology as an example.
FIG. 1a and FIG. 1b illustrate a device discovering process using Wi-Fi P2P. Hereinafter, a device-to-device communication device recognizing process based on Wi-Fi P2P technology standard that is a related art device-to-device communication scheme will be described.
A first P2P device 10 based on the Wi-Fi P2P technology standard may be activated by a first application unit 11 that is configured with an SME, an application, a user, and a vendor which correspond to the first P2P device 10. The activation is performed according to a device discovery command. When the device discovery command is received, the first P2P device 10 transits to a scan state 12 for searching for a candidate frequency channel, and search for all frequency channels. In FIG. 1a and FIG. 1b, it is assumed that frequency channels CH1, CH6 and CH11 are searched in the scan state 12.
Similarly to the first P2P device 10, a second P2P device 20 based on the Wi-Fi P2P technology standard may be activated by a second application unit 21 that is configured with an SME, an application, a user, and a vendor which correspond to the second P2P device 20. Also, similarly to the first P2P device 10, the second P2P device 20 may be activated according to a device discovery command. When the device discovery command is received, the second P2P device 20 transits to a scan state 22 for searching for a candidate frequency channel, and search for all the frequency channels. In FIG. 1a and FIG. 1b, it is assumed that the frequency channel CH6 is searched in the scan state 22.
Subsequently, the first P2P device 10 that has searched for the frequency channels in the scan state 12 transits to a listening state 13. The listening state 13 is a state in which the first P2P device 10 performs an operation of listening to whether a connection request message arrives from the other adjacent device, and a time of the listening state 13 is determined by using a value T1 which is arbitrarily selected. That is, the first P2P device 10 preferentially listens to a connection request signal for an arbitrarily selected time after the search of the frequency channels is completed in the scan state 12.
Moreover, the second P2P device 20 that has searched for the frequency channels in the scan state 22 transits to a listening state 23, and performs an operation of listening to whether a connection request message arrives from the other adjacent device.
When the first P2P device 10 that ends the search does not listen to the connection request message in the listening state 13, the first P2P device 10 transits to a search state 14. In the search state 14, the first P2P device 10 transmits the connection request signal for each of frequency channels that are acquired in the scan state 12.
In FIG. 1a and FIG. 1b, it is illustrated that the first P2P device 10 transmits the connection request signal at each of the frequency channels CH1, CH6 and CH11 that are acquired in the scan state 12, and performs an operation of searching for whether a connection response message arrives from the other adjacent P2P device. At this time, since the second P2P device 20 is in the listening state 23, the second P2P device 20 receives the connection request signal transmitted from the first P2P device 10 by using the frequency channel CH6, and in response to the connection request signal, the second P2P device 20 transmits a connection response message to the first P2P device 10.
The first P2P device 10 transmits the connection request signal for each of all the frequency channels that are acquired in the scan state 12, and checks whether the connection response message responding to the connection request signal arrives. In FIG. 1a and FIG. 1b, it is assumed that only the second P2P device 20 transmits the connection response message at the channel CH6.
Through the above-described process, the first application unit 11 of the first P2P device 10 finds the other adjacent P2P device, and establishes a communication link with the found P2P device.
However, in the communication link establishing process of the Wi-Fi P2P device, in addition to a Wi-Fi P2P device for establishing the communication link, a target Wi-Fi P2P device is in an activated state. Therefore, the Wi-Fi P2P device is targeted for communication at any time, and thus is activated for using the above-described scheme irrespective of an intention of establishing the communication link. For this reason, the communication link establishing process is very inefficient in terms of power consumption.
Moreover, in the communication link establishing process of the Wi-Fi P2P device, a Wi-Fi P2P device for establishing the communication link transits to the scan state 12, the listening state 13, or the search state 14. Here, a section length of the listening state 13 is arbitrarily selected, and thus, a collision of state occurs between the Wi-Fi P2P device and the other adjacent Wi-Fi P2P devices.
For example, when the first P2P device 10 is in the listening state 13 and the second P2P device 20 is in the listening state 23, only a time for establishing the communication link is consumed, and any operation is not performed. Also, when the first and second P2P devices 10 and 20 are in the respective search states 14 and 24 at the same time, the connection response message is not transmitted despite the connection request message being received, and for this reason, only a time for establishing the communication link is consumed.
Moreover, in the communication link establishing process of the Wi-Fi P2P device, the connection request message and the connection response message are received and transmitted in a state where a time is not synchronized and a gain is not adjusted, and thus, the communication link is established in a state where a quality of the communication link is degraded. Therefore, in a case of using communication link establishing process of the Wi-Fi P2P device, a length of a signal necessary to enhance the quality of the communication link increases, causing degradation in efficiency. In addition, when the length of the signal is reduced, a coverage in which the communication link is established is reduced because the quality of the communication link is degraded.
As described above, in devices that ensure mobility as in cellular phones, research is widely done for technology that reduces power consumption by using the sleep state. Also, devices that provide a connection of a wireless network such as Wi-Fi or the like do not perform a process of receiving an incoming call signal, but uses technology that reduces power consumption by using the sleep state.
However, research for reducing power consumption by using the sleep state is hardly done for devices in which power may be always supplied as in a device connected to a wired network or a device that is connected to a wireless network under an environment in which mobility is restricted. However, in devices (where power consumption in a standby state is very high) among devices where power may be always supplied as in set-top boxes, a necessity of reducing the power consumption in the standby state to increase the saving effect of energy is increasing.
Recently, devices such as set-top boxes support the wireless network such as Wi-Fi or the like in a state of being connected to the wired network such as Ethernet or the like, and thus, it is required to develop technology that reduces power consumption in the standby state by supporting the sleep state in a device connected to the wired network or the wireless network.
Therefore, it is required to develop a method and a system which reduce power consumption in the standby state and enables the transition between the sleep state and the active state without any time delay by effectively supporting the sleep state in a device accessible to the wired network or the wireless network.